Operating Systems
Operating systems like OS/2 1.x try to switch the processor between protected and real modes. This is both slow and unsafe, because a real mode program can easily crash a computer. OS/2 1.x defines restrictive programming rules allowing a Family API or bound program to run in either real or protected mode. Some early Unix operating systems, OS/2 1.x, and Windows used this mode. Windows 3.0 was able to run real mode programs in 16-bit protected mode. Windows 3.0, when switching to protected mode, decided to preserve the single privilege level model that was used in real mode, which is why Windows applications and DLLs can hook interrupts and do direct hardware access. That lasted through the Windows 9x series. If a Windows 1.x or 2.x program is written properly and avoids segment arithmetic, it will run the same way in both real and protected modes. Windows programs generally avoid segment arithmetic because Windows implements a software virtual memory scheme, moving program code and data in memory when programs are not running, so manipulating absolute addresses is dangerous; programs should only keep handles to memory blocks when not running. Starting an old program while Windows 3.0 is running in protected mode triggers a warning dialog, suggesting to either run Windows in real mode or to obtain an updated version of the application. Updating well-behaved programs using the MARK utility with the MEMORY parameter avoids this dialog. It is not possible to have some GUI programs running in 16-bit protected mode and other GUI programs running in real mode. In Windows 3.1 real mode disappeared.
Today, 16-bit protected mode is still used for running applications, e.g. DPMI compatible DOS extender programs (through virtual DOS machines) or Windows 3.x applications (through the Windows on Windows subsystem) and certain classes of device drivers (e.g. for changing the screen-resolution using BIOS functionality) in OS/2 2.0 and later, all under control of a 32-bit kernel.
Read more about this topic: Protected Mode
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